I dunno why.
Shoggoth Omelets
Yes, these are assassination weapons. Banned by every reputable, half-reputable, and disreputable Shadow Organization in the Great Work of Time, too. Get caught with one of these, and you’ll be lucky if you just end up with your brain in a jar.
How do they work? Well, you crack a ‘shoggoth egg,’ fry or scramble or poach the contents, and then feed it to your victim. It reportedly tastes great, although that’s probably not any kind of comfort when the Shoggoth Omelet abruptly turns you into a quivering pile of goo twelve hours later. Nobody knows if the process is painless, either: nobody’s ever taken the time to even scream for mercy when the transformation hits. Fortunately, the goo is not actually a shoggoth: people just call it that because it will kind of ooze around for a while and try to make sensory organs out of itself. And then the goo dies, and rapidly dessicates. As was stated before: assassination weapon.
So why is it banned? Because the stuff only gets activated when digested in a human body, but can survive in an active state for about a month or so. Nothing really kills it in the meantime, either – including dishwashers, detergent, bleach, or incinerators. That means that there’s a not-quite miniscule chance that a Shoggoth Omelet will claim more than one victim. The Secret Masters like using shared office kitchen facilities as much as the next entity; for that matter, many of them like to eat eggs. Coming down like a hammer from orbit on those that might take away those two things from them seems like a viable counter-terrorism strategy…